1. Field of the Invention
The present invention relates in general to analog circuit testers and in particular to a system for minimizing effects of switching power supply noise in an analog circuit tester.
2. Description of Related Art
A typical analog circuit tester includes a waveform generator for sending an analog test signal to an input of an analog device under test (DUT) and digitizer for digitizing an output signal produced by the DUT in response to the test signal. For example a tester may send a sequence of sine wave test signals of various frequencies to a DUT input and then digitize the DUT output signal to produce a data sequence representing the time varying behavior of the DUT output signal. The data sequence can then be analyzed using well-known digital signal processing techniques to determine such DUT characteristics as frequency response, signal-to-noise ratio, distortion and the like.
Noise generated by the analog circuit tester itself can influence test results. Tester-generated noise can enter the DUT as a component of the test signal or through air in the form of electromagnetic inductance and reappear as a component of the DUT output signal being digitized. The noise from the tester will then appear as a component of the digital output data sequence produced by the tester and will make such measured DUT parameters as signal-to-noise ratio and distortion look worse than they really are since a portion of the output signal noise is caused by the tester and not by the DUT.
Analog circuit testers typically employ line-powered switching power supplies because they can supply large amounts of power while being relatively small and efficient. A typical switching power supply is described at pages 360-368 of THE ART OF ELECTRONICS, second edition, a book by Paul Horowitz and Winfield Hill, published in the United States of America in 1989 by Cambridge University Press. Such a switching power supply rectifies an AC signal to produce a DC signal, pulse-width modulates a DC signal and then applies the pulse-width modulated DC signal to the primary of a transformer. The signal appearing at the transformer secondary is then rectified and filtered to produce the output power signal.
One problem with using a switching power supply is that it is a major source of noise, the act of pulse width modulating DC signal being a very noisy activity. Power supply noise is an obstacle to increasing the accuracy of analog circuit testing. The noise generated by a switching power supply can be reduced by filtering the output power signal and by shielding the power supply. But filtering and shielding does not completely eliminate noise. What is needed is a system for eliminating the effects of noise from a switching power supply on the measurements performed by an analog circuit tester employing such a power supply.